Method for Selecting One or More Transponders

ABSTRACT

A method and device for selecting one or more transponders, in particular backscatter-based transponders, from a plurality of transponders by a base station, which method is based on a slotted ALOHA method, in which the base station defines numbered time slots and a random number generated in a given transponder determines a time slot when the transponder transmits its transponder-specific identification to the base station. The random number is generated in a given transponder with the aid of a random number generator, the relevant random number generator is switched into a counter operating mode after reception of a selection command transmitted by the base station, while a count state of the random number generator is decremented or incremented when the base station transmits the start of a time slot, the relevant transponder transmits a transponder-specific identification to the base station if the count state of its random number generator is equal to a predetermined value, and the relevant random number generator is then switched back into the operating mode for random number generation.

This nonprovisional application claims priority under 35 U.S.C. §119(a)on German Patent Application No. DE102004041437.8-31, which was filed inGermany on Aug. 27, 2004, and which is herein incorporated by reference.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to a method for selecting one or moretransponders, in particular backscatter-based transponders, from aplurality of transponders by a base station.

2. Description of the Background Art

Selection methods, which are also called anticollision methods, aretypically used in, for example, contactless identification systems orradio frequency identification (RFID) systems. A system of this naturetypically has a base station or a reader and a plurality of transpondersor remote sensors, which are located in a response area of the basestation at the same time. If the data transmission is to take place onlybetween one transponder or a group of transponders and the base station,a selection process must be carried out prior to the data transmissionin question.

In this context, a basic distinction is made between stochastic anddeterministic selection methods. A detailed description of deterministicselection methods and also stochastic selection methods can be found,for example, in the textbook by Klaus Finkenzeller, RFID-Handbuch,3^(rd) edition, HANSER, 2002, see especially Chapter 7.2,Vielfachzugriffsverfahren (multiple access methods), which has beenpublished in English by John Wiley & Sons, and which is incorporated byreference herein.

In contrast to deterministic methods, stochastic methods do notpresuppose a unique identification (U-ID) with a structure such as thosedescribed in the ISO 15963 standard. Assignment of such U-IDs isundertaken by bodies including a variety of manufacturer-independentorganizations, for example the EAN/UCC or the IATA. However, theassignment can also be made by a manufacturer on its own. As a result,it is not always possible to ensure the uniqueness of U-IDs in opensystems in which transponders from arbitrary manufacturers may belocated in the response area of a base station. Stochastic methodspermit selection even in these cases. Examples of such stochasticmethods include the ALOHA method, the slotted ALOHA method, and thedynamic slotted ALOHA method.

The ALOHA method is a transponder-controlled, stochastic method in whichthe transponders transmit their data for transmission with a timeoffset. As a rule, the time offset is set on the basis of a randomnumber generated in the transponder. If multiple transponders transmitan identification within the same time slot, a so-called collisionoccurs. This generally prevents the base station from being able toreceive the transmitted data error-free.

In the slotted ALOHA method, the probability of collision issignificantly reduced as compared to the plain ALOHA method. It is abase-station controlled, stochastic method in which the transponders areactive, i.e. begin transmission of data, only at defined, synchronouspoints in time. To this end, the base station prescribes numbered timeslots, or slots, and the transponders each generate a random number,with every transponder whose random number corresponds to the number ofa time slot transmitting data or an identification to the base stationin this time slot. To initiate the selection process, the base stationgenerally transmits a command to the transponders, which indicates thestart of a selection procedure. After receiving the command, thetransponders store the applicable random numbers, which for example werepreviously generated or calculated in the transponder. When only onetransponder transmits an identification within a time slot, thistransponder is selected within the time slot, or can be selected by thebase station by transmission of a command or an acknowledgement signal.The base station can then, for example, perform write and/or readoperations on this transponder.

When multiple transponders transmit an identification within the sametime slot, a collision occurs. Depending on the bit coding, the basestation can detect such a collision immediately or after a delay, andcan skip the corresponding time slot and attempt to process time slotsin which no collision occurs, or can initiate a new selection procedureby sending an appropriate command to the transponders. Since thetransponders typically generate or store new random numbers, thepossibility exists that no collision will now occur.

The probability of collision depends on the number of transponders inthe base station's response area and the number of time slots madeavailable. Since the number of transponders can fluctuate tremendously,a static number of time slots can lead to problems. If the number oftime slots is too small, the probability of collision increases sharply.If the number of time slots is too large, there are correspondingly manytime slots in which no transponder transmits data. The time required forthe selection process thus increases sharply in both cases. To achieveoptimum throughput, the number of time slots in which the transponderstransmit data should be selected to approximately equal the number oftransponders.

The dynamic slotted ALOHA method, in which the number of available timeslots can be controlled by the base station, was created in order tosolve this problem. In this method, the base station can initiate aselection process with a small number of time slots, for example. Ifcollisions frequently occur in this case, the base station can initiatea new selection process in which the number of time slots is increased,thus reducing the probability of collisions.

A variety of methods are known for producing a random number for thestochastic methods. Thus, for example, the time period between a resetof the transponder and the point in time when a first symbol is receivedcan be used as a basis for calculating the random number. Other methodscombine numbers from two different areas of memory in order to determinethe random number, while as a further refinement, a received data itemcan additionally be included in the calculation.

Other methods use a linear feedback shift register for random numbergeneration; the shift register can be operated with a clock source,which has a certain amount of dispersion between different transponders,for example. As a result of their individual clock sources, after acertain operating time, the shift registers of different transpondersthen exhibit different values which can be used as random numbers.

In the slotted ALOHA method, the base station defines numbered timeslots and a transponder whose random number corresponds to the number ofa time slot sends data or identification to the base station during thistime slot.

For this purpose, a transponder customarily has what is known as a slotcounter and a binary comparator in addition to the random numbergenerator. After the initiation of the selection process by the basestation, the slot counter is decremented or incremented, starting froman initial value, when the base station indicates the start of a newslot or time slot by transmitting a corresponding command. The binarycomparator compares the random number present in the random numbergenerator with the current slot number of the slot counter, and if therandom number and slot number match, the relevant transponder transmitsits identification to the base station. Since the random numbergenerator and the slot counter are designed as separate units, such animplementation requires a relatively large chip area.

SUMMARY OF THE INVENTION

It is therefore an object of the present invention to provide a methodand a device for abase station to select one or more transponders from aplurality of transponders that permits reliable and time-efficientselection and requires comparatively little chip area for implementationin a transponder.

In accordance with the invention, the random number can be generated ina given transponder with the aid of a random number generator. Therandom number generator is switched into a counter operating mode afterreception of a selection command transmitted by the base station. In thecounter operating mode, the random number generator operates as a normalcounter or slot counter. The count state of the random number generatoroperating as a counter is decremented or incremented when the basestation transmits the start of a time slot. If the count state of therandom number generator is equal to a predetermined value, the relevanttransponder transmits a transponder-specific identification to the basestation. The relevant random number generator is then switched back intothe operating mode for random number generation. As a result of the factthat the random number generator also serves as a slot counter duringcertain time intervals, it is possible to eliminate a separate slotcounter used only for this purpose. This reduces the chip area needed.Due to the switchover of the random number generator to the operatingmode for random number generation after the transmission of thetransponder-specific identification, the generation of a new randomnumber begins as early as possible. In the case of random numbergeneration based on a dispersion of the clock source, this ensuresoptimal utilization of the random number space, since the differences inthe clock sources of different transponders have a greater effect as aresult of the longer time duration of random number generation. Whenanother selection process is to be performed after the current selectionprocess, for example because collisions have occurred in a slot, usablerandom numbers are thus available soon in the transponders in question,with the result that no waiting time is necessary between successiveselection processes.

In a further embodiment, the random number can be generated with the aidof a linear feedback shift register. Such shift registers can easily beswitched between the operating mode for generating the random number andthe counter operating mode. Moreover, it is simple to generate randomnumbers with the aid of such components in combination with a dispersionor uncertainty of a clock source used to clock the shift register.

In yet a further embodiment, switchover into the operating mode forgenerating random numbers can depend on whether the base stationtransmits a command to the transponder following the transmission of thetransponder-specific identification. For example, switchover into theoperating mode for generating random numbers can be omitted if the basestation sends an acknowledgement command to the transponder in question.The selection of the transponder is indicated by the acknowledgementcommand, i.e., it need not necessarily participate in a subsequentprocess. In this case, the immediate switchover into the operating modefor generating random numbers can be omitted, which reduces the powerconsumption of the transponder and thus increases its transmissionrange.

In another embodiment, an initial count state of the random numbergenerator can be the generated random number, and the relevanttransponder transmits its transponder-specific identification to thebase station when the count state of its random number generator iszero. The random number generator merely switches to the counteroperating mode here and then decrements or increments the count state.Consequently, no separate storage element for storing the random numberis necessary, thus reducing the chip area needed. Detection that thecount state has assumed the value ZERO is simpler to implement incircuit technology than a comparison with an arbitrary value. This inturn reduces the necessary chip area.

The generated random number can be stored, then the count state of therandom number generator can be set to an initial value, in particularzero, and the transponder in question transmits its transponder-specificidentification to the base station when the count state is equal to thestored value of the random number.

Further scope of applicability of the present invention will becomeapparent from the detailed description given hereinafter. However, itshould be understood that the detailed description and specificexamples, while indicating preferred embodiments of the invention, aregiven by way of illustration only, since various changes andmodifications within the spirit and scope of the invention will becomeapparent to those skilled in the art from this detailed description.

BRIEF DESCRIPTION OF THE DRAWINGS

The present invention will become more fully understood from thedetailed description given hereinbelow and the accompanying drawingswhich are given by way of illustration only, and thus, are not limitiveof the present invention, and wherein:

FIG. 1 is a block diagram of a transponder having a random numbergenerator—that is operated in an operating mode for generating randomnumbers or in a counter operating mode—, a control logic unit, and azero detection unit;

FIG. 2 is a timing diagram of control signals of the units from FIG. 1during a selection process; and

FIG. 3 is a state diagram of the transponder from FIG. 1 during theselection process shown in FIG. 2.

DETAILED DESCRIPTION

FIG. 1 shows a block diagram of a backscatter-based, passive transponderTR with a control logic unit SE, a random number generator ZG thatoperates either in an operating mode for generating random numbers or ina counter operating mode as a function of a signal RC provided by thecontrol logic unit SE, and a zero detection unit NE that provides asignal SA, which signal enables the transmission of atransponder-specific identification to a base station.

In the operating mode for generating random numbers, the random numbergenerator ZG is configured as a linear feedback shift register that isoperated with a clock provided in the transponder TR. The clock providedin the transponder TR exhibits a certain dispersion between differenttransponders, by which means different values arise in the shiftregisters of the transponders in question after a certain operatingperiod. In the counter operating mode, the random number generator ZG isconfigured as a conventional counter, for example as a ripple counter.

The zero detection unit NE is coupled to the random number generator ZGand monitors the count state of the random number generator ZG when thelatter is in the counter operating mode. If the count state of therandom number generator ZG is zero, the signal SA becomes active, thusactivating the transmission of the transponder-specific identificationto the base station.

FIG. 2 shows a timing diagram of control signals of the units shown inFIG. 1 during a selection process by a base station BS. The selectionmethod or selection process, in this example, is based on a slottedALOHA selection method or on a dynamic slotted ALOHA selection method.

The base station initiates the selection process by transmitting aselection command AK. The transponder TR receives the selection commandAK. The control logic unit SE then activates the signal RC by a leveltransition from low to high. As a result of the level transition of thesignal RC, the random number generator ZG is switched over from theoperating mode for generating random numbers to the counter operatingmode by clocking of the linear feedback shift register. During thecounter operating mode, a count state ZS of the random number generatoris decremented when the base station transmits the start of a time slot.The initial count state ZS here is the random number contained in theshift register prior to the switchover into the counter operating mode.In the example embodiment shown, the initial count state ZS is 3.

The base station now transmits a command NS, which indicates the startof a time slot. In response, the count state ZS of the random numbergenerator ZG is reduced by one, to 2. The zero detection unit NE checkswhether the count state ZS is zero. Since this is not yet the case, thesignal SA remains at a low level.

As a result, the base station BS now transmits two additional commandsNS, with the count state ZS again being reduced by one in each case.After the last command NS, the count state ZS is zero. The zerodetection unit NE detects this, and activates the signal SA by a leveltransition from low to high. In response, the transponder TR transmits atransponder-specific identification ID to the base station BS.

The base station BS receives the transponder-specific identification IDand sends an acknowledgement signal QS to the transponder TR. Thetransponder TR is now selected in the current time slot, which isindicated by a signal SEL. The base station BS can then, for example,perform write and/or read operations (not shown) on the transponder TR.

Following the transmission of the transponder-specific identification IDby the transponder TR, the control logic unit SE deactivates the signalRC through a level transition from high to low. As a result of the leveltransition of the signal RC, the random number generator ZG is againswitched over to the operating mode for generating random numbers byclocking of the linear feedback shift register, with an initial value Xbeing assigned to the shift register. This switchover could also beomitted in the case shown, since the base station BS transmits theacknowledgement signal QS.

Once the necessary operations with the transponder TR have beencompleted, the base station BS transmits an additional command NS,indicating a new time slot in which other transponders (not shown) canbe selected. The transponder shown then withdraws from the currentselection process.

FIG. 3 shows a state diagram of the transponder from FIG. 1 during theselection process shown in FIG. 2. At the start of the selectionprocess, prior to reception of the command AK, the transponder TR is ina base state Z1. In this state Z1, the random number generator ZGcontinuously generates random numbers.

When the transponder TR receives the command AK sent by the base stationBS, it switches to a state Z2. In the state Z2, the random numbergenerator ZG is switched over to the counter operating mode. The initialcount state ZS now is the random number contained in the shift registerprior to the switchover to the counter operating mode. The transpondernow waits to receive a command NS.

When the transponder TR receives the command NS sent by the base stationBS, it switches to a state Z3 and decrements the count state ZS. In thestate Z3, the count state ZS is decremented by one each time a commandNS is received from the base station.

When the count state ZS is zero, the transponder TR switches to a stateZ4, in which it transmits its transponder-specific identification ID tothe base station BS. The random number generator ZG is then againswitched over into the operating mode for random number generation. Thetransponder now waits to receive the acknowledgement command QS from thebase station BS.

When the transponder TR receives the acknowledgement command QS sent bythe base station BS, it switches to a state Z5 during which it isselected. The base station BS can then, for example, perform writeand/or read operations (not shown) on the transponder TR.

When the transponder TR receives another command NS sent by the basestation BS, it switches back to the state Z1, which is to say itwithdraws from the current selection process and waits for a newselection command AK.

Naturally, the diagrams shown are merely examples, and serve only todemonstrate the inventive selection process by way of example.

As an alternative to the example embodiment shown, the generated randomnumber can be stored after the command AK is received, with the countstate of the random number generator now functioning as a counter thenbeing set to an initial value, in particular zero. As in the illustratedexample embodiment, the count state is also incremented with eachcommand NS. The transponder transmits its transponder-specificidentification to the base station when the count state is equal to thestored value of the random number.

The embodiment shown reduces the number of components needed for aslotted selection process, making it possible to reduce the chip arearequired. In addition, the available random number space is betterutilized as a result of the early switchover to renewed random numbergeneration. This permits an additional selection process immediatelythereafter, if such is necessary.

The invention being thus described, it will be obvious that the same maybe varied in many ways. Such variations are not to be regarded as adeparture from the spirit and scope of the invention, and all suchmodifications as would be obvious to one skilled in the art are to beincluded within the scope of the following claims.

1. A method comprising, by one or more computer systems, selecting oneor more transponders from a plurality of transponders.